Microbial Growth Kinetics

Lecture 2

Fermentation Process

Types of Culture

• Fermentation - carried out as batch, continuous and fed-batch

• Dictated largely by type of product being produced

Batch Culture

• Closed culture system – initially contains limited amount of nutrient

• No growth – lag phase – time of adaptation

• Growth rate increases – grow constantly at maximum rate – Log or exponential phase

Batch Culture- Exponential phase/Trophophase

• Exponential phase equation: dx/dt = µx

where x is the concentration of microbial biomass, t is time in hours and µ is the specific growth rate in hours

• Nutrients are in excess and organism is growing at its maximum specific growth rate = µmax

Batch Culture – Deceleration phase

• Cessation of growth – due to depletion of some essential nutrient in medium (substrate limitation)

• Accumulation of some autotoxic product of organism in medium (toxin limitation)

• Or combination of both

Batch Culture – Stationary phase/Idiophase

• Point where growth rate has declined to zero.

• Population is metabolically active – producing secondary metabolites

• Maximum population phase

What is Yield Factor (Y)?

• It is a measure of efficiency of conversion of any one substrate into biomass.

• It is not constant – varies according to growth rate, pH, temperature, the limiting substrate and concentration of substrates in excess.

How can deceleration phase be tested?

• Zone A to B – increase in initial substrate concentration – proportionally increases biomass produced at stationary phase

• x=Y(Sr-s) where x is concentration of biomass produced

Y is yield factor, Sr is initial substrate con. and s is residual conc.

Growth kinetics of Metabolites

• Growth-linked products/Primary metabolites• Formation of growth linked product is described by equation- dp/dt = qpx where p is the concentration of the product- qp is specific rate of product formation (mg product/g

biomass/h)- Product formation is related to biomass production by

equation dp/dx = Yp/x- Where Yp/x is the yield of product in terms of biomass (g

product/g biomass)

Batch fermentation – used for?

• Producing biomass – fastest growth rate and maximum cell population

• Primary metabolites – extend exponential phase

• Secondary metabolites – decreased growth in log phase

Continuous Culture (Chemostat)

• Addition of fresh medium to the vessel – exponential growth

• Medium is substrate limited • Overflow device – added medium displaces

equal volume of culture – continuous production of cells

- Formation of new biomass balanced by loss of cells from vessel

Chemostat culture

• Chemostat culture – Cells and spent medium are continuously removed

- State of culture is dependent upon flow rate of fresh medium

Continuous Culture (Chemostat)

• Dilution rate - The flow of medium into the vessel is related to volume of the vessel

D = F/V- F is flow rate (dm3/h)- V is volume (dm3)- D is dilution rate in per hour- Net change in cell concentration over a time

period = dx/dt = growth (µx) – output (Dx)

Problems - Chemostat

• Problems – Imperfect mixing and wall growth- Imperfect mixing – increase in degree of

heterogeneity in fermenter- Wall growth – Organism adheres to inner

surfaces of reactor – increases heterogeneity- Limited by coating inner surfaces of vessel

with Teflon

Feedback systems - Chemostat

• Internal feedback – Limiting exit of biomass

• External feedback – Subjecting effluent stream to biomass separation

Perfusion/Turbidostat

• Perfusion culture

- Medium is pumped continuously - Cells are retained- Becoming popular for large-scale production- Attains high cell density- Cell separator

Continuous culture

• Biomass production

• Microbial biomass produced for human or animal consumption – Single Cell Protein (SCP)

• Practice 1960s

What is Fed-batch culture?

• In open system/Fed-batch culture – involves controlled nutrient feeding

• Partial media changes at regular intervals

Fed-batch culture

• Initial batch cultures – fed continuously or sequentially with medium

• No removal of culture fluid

• Three types – Variable volume, Fixed volume and Cyclic fed-batch

Fed-batch culture

• Batch culture is fed in following ways- Same medium + conc. used to establish batch

culture is added –Variable volume- Conc. solution of limiting substrate is added

at a rate less than initial- Very conc. Solution of limiting substrate at

lesser rate than initial – Fixed volume

Cyclic fed-batch culture

• Life of variable fed-batch fermentation may be extended

- Withdrawing a portion of culture/residual culture

- Increase in dilution rate and specific growth rate

Fed-batch culture

• Used for biomass

• Primary metabolite

• Secondary metabolite

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